Green fluorescence protein (GFP) is a 27 KDa protein derived from the bioluminiscent jellyfish Aquorea victoria, emiting green light (509 nm) when excited (excitation by Blue or UV light, absorption peak at 395 nm).GFP is a useful tool in cell biology research, as its intrinsic fluorescence can be visualized in living cells. Light-stimulated GFP fluorescence is species-independent and a fluorescence has been reported from many different types of GFP-expressing hosts, including microbes, invertebrates, vertebrates and plants. No exogenous substrates and cofactors are required for the fluorescence of GFP, since GFP autocatalytically forms a fluorescent pigment from natural amino acids present in the nascent protein.GFP fluorescence is stable under fixation conditions and suitable for a variety of applications. GFP is widely used as a reporter (tag) for gene expression, enabling researchers to visualize and localize GFP-tagged proteins within living cells without any further staining. Other applications of GFP include measurement of distance between proteins through fluorescence energy transfer (FRET) protocols.To increase a fluorescence intensity of GFP, chromophore mutations have been created. The Enhanced GFP has a fluorescence 35 times more intense than the wt-GFP. Mutagenesis of GFP has produced also many mutants (e.g. Yellow Fluorescent Protein, Cyan Fluorescent Protein) with warying spectral properties. Antibodies raised against full-length GFP variants should also detect other variants of the protein.